Loading…
Voltage, current and resistance
The three quantities every circuit is about.
The three quantities every circuit is about.
Every circuit, from a flashlight to a transceiver, comes down to three quantities:
Nothing in a circuit happens without all three. The next article, Ohm's law, ties them into one equation.
Picture a closed loop of pipe with a pump. The pump supplies pressure; the water that moves is the flow rate; a narrow section of pipe resists the flow. Change any one and the others respond: more pressure gives more flow, and a narrower pipe gives less.
The analogy also shows two things beginners often miss:
Voltage, current and resistance are measured and described differently:
| Quantity | Applies | You say | Meter hookup |
|---|---|---|---|
| Voltage | between two points | "across" the part | voltmeter in parallel |
| Current | along a path | "through" the part | ammeter in series |
| Resistance | a property of the part | "of" the part | ohmmeter, power off |
"The voltage through a resistor" is the classic slip. Use voltage across, current through. See Multimeters for how the hookups work, and Series and parallel circuits for what "in series" and "in parallel" mean.
Current is the flow of electrons through a conductor, but by convention it is drawn as flowing from the positive terminal to the negative one, the opposite way to the electrons. The convention was set before electrons were discovered; every equation and schematic arrow uses it, so use it too.
Electrons themselves drift very slowly. In a 1 mm² copper wire carrying 1 A they creep along at about 0.07 mm per second. A lamp lights at once because the electric field that pushes them travels through the wire at a large fraction of the speed of light, moving all the electrons together, like a pipe already full of water.
Resistance turns electrical energy into heat, which is the subject of Power and energy. What gives a material its resistance is covered in Conductors and insulators.